This invention relates to semiconductor devices, and more specifically relates to a novel article and process for manufacturing an alloyed diode.
Semiconductor devices are commonly made by either alloying or diffusing a junction into a silicon wafer. Devices made by the alloyed process are less expensive than those made by the diffusion process since the manufacturing equipment and process is much simpler. The alloyed device, however, usually has a lower blocking voltage than those made by a diffusion process but this blocking voltage may still be as high as about 1200 volts. Thus, devices made by an alloying process are suitable for many applications such as in power supplies for welders and the like where relatively low voltage devices can be used, and where cost is a principal element in the choice of the device to be used.
The process now used for the manufacture of an alloyed diode requires the handling and stacking of a large number of thin foils. Foils are very difficult to handle and are virtually impossible to be used in an automated process. Thus, a great deal of hand labor is necessary.
In the conventional process, a thin aluminum foil is stacked against one surface of an N type silicon wafer. At the same time, the molybdenum electrodes are stacked on the opposite surfaces of the silicon. Very thin foils of material which enable soldering to the molybdenum members are placed between one molybdenum member and the silicon and on opposite surfaces of the molybdenum. All members are secured together and the aluminum is alloyed into the silicon during a heating operation.
The thin solder foils may be of any desired type and typically can be foils of material known as a "quad" material which is an alloy consisting principally of silver containing about 2.9% by weight of germanium, 5% by weight of copper and 0.05 to 0.1% by weight of arsenic. These quad foils are disposed on the outer surfaces of the molybdenum and are disposed between the bottom molybdenum electrode and the bare silicon surface on the opposite side of the surface receiving the aluminum layer.
The quad foils which are used in this process are very thin, for example, 1 to 2 mils. Similarly the aluminum is thin and may be about 4.5 mils thick. These foils are not easily handled and the stack of the numerous foils along with the molybdenum expansion electrodes and the silicon wafer cannot be easily automated.